1. Field of the Invention
The present invention relates to a driving mechanism of a backlight module, and more particularly, to a luminance-adjusting driving circuit and related method of a backlight module using a hot cathode fluorescent lamp (HCFL).
2. Description of the Prior Art
For a display apparatus having a backlight module, such as a liquid crystal display (LCD), an appropriate luminance-adjusting mechanism is required for adjusting the luminance of a backlight source due to the considerations of an ambient light intensity and a user's preferences.
When a hot cathode fluorescent lamp (HCFL) serves as the backlight source, a frequency modulation control, an amplitude modulation control, or a pulse width modulation (PWM) control is generally used as the luminance-adjusting method of a driving circuit. A driving circuit for performing the frequency modulation control is easy to design, and is able to adjust the luminance of the backlight source efficiently. However, because of a frequency variation of a control signal of this driving circuit, a design of a front-end filter is difficult due to the electro-magnetic interference (EMI), and magnetic components cannot be optimally applied in the driving circuit. Furthermore, the amplitude modulation control adjusts the luminance by changing a DC current of a resonant circuit, and the design of the driving circuit is more difficult. The PWM control adjusts the luminance by adjusting an enabling period of a switch. Generally, a symmetrical PWM control is used as the PWM control, although the driving circuit of the PWM control is more complex than that of the frequency modulation control, and has a higher power consumption because of switching operations.
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a prior art quasi-half-bridge frequency-varied driving circuit 100. The driving circuit 100 includes a DC current source Vdc, a signal generator 110, a resonant circuit 120 coupled to the signal generator 110, a capacitor 140 coupled to the resonant circuit 120 and a backlight source 130, and two capacitors 160 and 170 coupled to the signal generator 110 and the backlight source 130. The signal generator 110 is used for generating an alternating current (AC) signal having a variable frequency. The resonant circuit 120 is used for generating an oscillation signal to drive the backlight source 130 according to the AC signal. The capacitor 140 is used to provide an impedance to adjust a current value of the backlight source 130. The capacitors 160 and 170 are used to generate a DC voltage level. In addition, the signal generator 110 includes two transistors 112 and 114, and the frequency of the AC signal can be determined by adjusting a frequency of switching on/off the transistors 112 and 114. The resonant circuit 120 includes an inductor 122 and a capacitor 124, which is used to convert the AC signal generated from the signal generator 110 to a sinusoidal wave to drive the backlight source 130.
As shown in FIG. 1, the capacitor 140 is connected in parallel to the backlight source 130. When the AC signal generated from the signal generator 110 has a frequency ω, the impedance of the capacitor 140 is (1/ωCf), where Cf is a capacitance of the capacitor 140. Then, the current of the backlight source 130 is determined according to a ratio between the impedance of the capacitor 140 and an impedance of the backlight source 130. When the impedance of the capacitor 140 is greater than the impedance of the backlight source 130, the backlight source 130 is in the main current path and the backlight source 130 lightens; and when the impedance of the capacitor 140 is less than the impedance of the backlight source 130, the capacitor 140 is in the main current path and the luminance of the backlight source 130 is degraded or even extinguished.
A circuit structure of the above-mentioned luminance-adjusting method is simple, however, the front-end filter will be interfered with by the electro-magnetic wave due to the frequency variation, and the magnetic components cannot be optimally applied in the driving circuit.